The present invention relates generally to variable displacement pumps, and more particularly to variable displacement vane pumps useful for automotive continuously variable transmissions and automotive power steering systems.
Japanese Patent Application Publication No. 7-119648 discloses a variable displacement vane pump for an automotive power steering system. This variable displacement vane pump generally includes a pump body, an adapter ring, a cam ring, a rotor, and a pressure plate. The pump body includes a front body and a rear body which are coupled to form a chamber therebetween. Specifically, the front body includes a cylinder portion having an inner space extending longitudinally therethrough, and a base portion covering a first longitudinal end of the inner space of the cylinder portion, while the rear body covers a second longitudinal end of the inner space of the front body. The adapter ring is mounted in the inner space of the pump body, and fitted and fixed to the inner radial periphery of the front body, defining an elliptical space inside. The cam ring is mounted in the elliptical space for moving laterally leftward and rightward. The rotor is mounted inside the cam ring, and fixed to a drive shaft extending through the front body of the pump body. The pressure plate is disposed between the rotor and the base portion of the front body, and in sliding contact with one longitudinal end surface of the rotor. The rotor includes a plurality of slots arranged circumferentially at the outer radial periphery, the slots extending radially of the rotor. A plurality of vanes are mounted in respective ones of the slots for moving longitudinally thereof. The vanes separate the space defined between the rotor and the cam ring, defining a plurality of pump chambers. The space defined between the cam ring and the rotor includes a first region in which each pump chamber gradually expands with rotation of the rotor, and a second region in which each pump chamber gradually contracts with rotation of the rotor. A suction port is formed in one longitudinal end surface of the rear body so as to face the first region, while a discharge port is formed in one longitudinal end surface of the pressure plate so as to face the second region. A pressure relief groove is defined in a portion of the inner radial periphery of the cam ring between the first region and the second region for allowing the working fluid to flow between two adjacent pump chambers. This is intended for allowing the working fluid to flow from the pump chamber in the second region to the pump chamber in the first region, preventing the internal pressure of the pump chamber from rapidly changing when the pump chamber moves from the first region to the second region, and thereby reducing fluctuations in the discharge pressure of the variable displacement vane pump.